$ \left(\dfrac{49}{25}\right)^{-\frac{1}{2}}$
Explanation: $= \left(\dfrac{25}{49}\right)^{\frac{1}{2}}$ Figure out what goes in the blank: $\Big(? \Big)^{2}=\dfrac{25}{49}$ Figure out what goes in the blank: $\Big({\dfrac{5}{7}}\Big)^{2}=\dfrac{25}{49}$ So $\left(\dfrac{49}{25}\right)^{-\frac{1}{2}}=\left(\dfrac{25}{49}\right)^{\frac{1}{2}}=\dfrac{5}{7}$